Mechanical ventilatory support is widely accepted as an effective form of therapy and means for treating patients with respiratory failure. Ventilation is the process of delivering oxygen to and washing carbon dioxide from the alveoli in the lungs. When receiving ventilatory support, the patient becomes part of a complex interactive system that is expected to provide adequate ventilation and promote gas exchange to aid in the stabilization and recovery of the patient. Clinical treatment of a ventilated patient often calls for monitoring a patient's breathing to detect an interruption or an irregularity in the breathing pattern, for triggering a ventilator to initiate assisted breathing, and for interrupting the assisted breathing periodically to wean the patient off of the assisted breathing regime, thereby restoring the patient's ability to breathe independently.
In those instances in which a patient requires mechanical ventilation due to respiratory failure, a wide variety of mechanical ventilators are available. Most modern ventilators allow the clinician to select and use several modes of inhalation either individually or in combination via the ventilator setting controls that are common to the ventilators. These modes can be defined in three broad categories: spontaneous, assisted or controlled. During spontaneous ventilation without other modes of ventilation, the patient breathes at his own pace, but other interventions may affect other parameters of ventilation including the tidal volume and the baseline pressure, above ambient, within the system. In assisted ventilation, the patient initiates the inhalation by lowering the baseline pressure by varying degrees, and then the ventilator “assists” the patient by completing the breath by the application of positive pressure. During controlled ventilation, the patient is unable to breathe spontaneously or initiate a breath, and is therefore dependent on the ventilator for every breath. During spontaneous or assisted ventilation, the patient is required to “work” (to varying degrees) by using the respiratory muscles in order to breathe.
The total work of breathing (the work to initiate and sustain a breath) performed by a patient to inhale while intubated and attached to the ventilator may be divided into two major components: physiologic work of breathing (the work of breathing of the patient, “WOBp”) and breathing apparatus (endotracheal tube and ventilator) imposed resistive work of breathing or imposed work of breathing (“WOBi”). The total work of breathing (“WOB”) can be measured and quantified in Joules/L of ventilation. In the past, techniques have been devised to supply ventilatory therapy to patients for the purpose of improving patient's efforts to breathe by decreasing the total work of breathing to sustain the breath. Still other techniques have been developed that aid in the reduction of the patient's inspiratory work required to trigger a ventilator system “ON” to assist the patient's breathing. It is desirable to reduce the effort expended by the patient in each of these phases, because a high total work of breathing load can cause further damage to a weakened patient or be beyond the capacity or capability of small or disabled patients.
Furthermore, it is desirable to quantify the imposed work of breathing (WOBi) of a patient since this value is critical in the decision of when to extubate or remove ventilatory support from the patient. High work loads tend to create breathing patterns that are rapid and shallow (high frequency and low tidal volume). Rapid shallow breathing is typically a contra-indicator of extubation success. If this high work load, however, is caused mainly by a large imposed work of breathing such that most of the excess work is caused by the breathing apparatus, extubation success is much higher (“Elevated imposed work of breathing masquerading as ventilator weaning intolerance.” Chest. 1995 October; 108(4): 1021-5).
The early generation of mechanical ventilators, prior to the mid-1960s, were designed to support alveolar ventilation and to provide supplemental oxygen for those patients who were unable to breathe due to neuromuscular impairment. Since that time, mechanical ventilators have become more sophisticated and complicated in response to increasing understanding of lung pathophysiology. In an effort to improve a patient's tolerance of mechanical ventilation, assisted or patient-triggered ventilation modes were developed. Partial positive pressure ventilation (PPV) support, in which mechanical support supplements spontaneous ventilation, became possible for adults outside the operating room when intermittent mandatory ventilation (IMV) became available in the 1970s. Varieties of “alternative” ventilation modes addressing the needs of severely impaired patients continue to be developed.
In recent years, microprocessors have been introduced into modern ventilators. Microprocessor ventilators are typically equipped with sensors that monitor breath-by-breath flow, pressure, volume, and derive mechanical respiratory parameters. Their ability to sense and transduce “accurately,” combined with computer technology, makes the interaction between clinician, patient, and ventilator more sophisticated than ever. The prior art microprocessor controlled ventilators suffered from compromised accuracy due to the placement of the sensors required to transduce the data signals. Consequently, complicated algorithms were developed so that the ventilators could “approximate” what was actually occurring within the patient's lungs on a breath-by-breath basis. In effect, the computer controlled prior art ventilators were limited to the precise, and unyielding, nature of the mathematical algorithms that attempted to mimic cause-and-effect in the ventilator support provided to the patient.
The overall performance of the assisted ventilatory system is determined by both physiological and mechanical factors. The physiological determinants, which include the nature of the pulmonary disease, the ventilatory efforts of the patient, and many other physiological variables, changes with time and are difficult to diagnose. Moreover, the physician historically had relatively little control over these determinants. Mechanical input to the system, on the other hand, is to a large extent controlled and can be reasonably well characterized by examining the parameters of ventilator flow, volume, and/or pressure. Optimal ventilatory assistance requires both appropriately minimizing physiologic workloads to a tolerable level and decreasing imposed resistive workloads to zero. Doing both should ensure that the patient is neither overstressed nor oversupported. Insufficient ventilatory support places unnecessary demands upon the patient's already compromised respiratory system, thereby inducing or increasing respiratory muscle fatigue. Excessive ventilatory support places the patient at risk for pulmonary-barotrauma, respiratory muscle deconditioning, and other complications of mechanical ventilation.
In addition to total work of breathing (WOB), there are other measurements of patient effort including power of breathing (POB), the rate at which total work of breathing is done, and the pressure time product (PTP), the integrated product of time multiplied by the decrease in pleural pressure during a breath. These methodologies are similar in their goal of measuring patient effort, but are calculated differently and provide different measures of the patient effort.
Although total work of breathing (and its alternatives) has been considered an important parameter for appropriately setting a ventilator, it has remained largely unused because of the difficulty in obtaining its value. Physiologic work of breathing is defined using a pleural pressure versus volume graph of a patient's breath. The pleura is a two-layered membrane that envelops the lung and contains lubricating fluid between its inner and outer layers. During breathing, the respiratory muscles either compress or expand the lungs by exerting forces on the pleura. The pressure in the pleural space therefore represents the respiratory effort. The patient's physiologic work of breathing is the area from the chest wall compliance line on the right to the pleural pressure versus volume loop on the left (see FIG. 1). Since the pleural pressure is very difficult to obtain and may be different at different positions in the pleural space, a typical surrogate for pleural pressure is esophageal pressure. The esophageal pressure is typically obtained by placing a balloon ill the esophagus between the heart and the stomach.
Likewise, although imposed work of breathing has been considered an important parameter for appropriately setting a ventilator, it has also remained largely unused because of the difficulty in obtaining its value. Imposed work of breathing is the area below baseline pressure circumscribed within the tracheal pressure-tidal volume loop during spontaneous inhalation. Typically, this is done by using a catheter inserted into the tracheal tube or a lumen in the side of the tracheal tube that opens at the distal end of the tracheal tube. These devices are then attached to a pressure transducer to measure tracheal pressure. The greatest single difficulty with these devices is the harsh environment in which they exist and their propensity for becoming clogged. For this and other reasons, tracheal pressure is difficult to reliably measure and thus is not normally used.
U.S. Pat. No. 5,316,009 describes an apparatus for monitoring respiratory muscle activity based on measuring resistance and elastance of the lung and then calculating a value called Pmus from the standard equation of motion where Paw=Pmus+R*flow+V/C. It also discloses calculation of a PTP of pmus, which is not the standard PTP, and a “work” Wmus, but not necessarily real WOB. A problem with the method taught by the '009 patent is that Pmus is difficult to measure in a spontaneously breathing patient because the parameters R and C must be very accurately computed in order for Pmus to correlate with “work”. Moreover. R and C in a spontaneously breathing patient with ventilator support are very difficult to obtain accurately.
Occlusion pressure at 0.1 seconds after breath initiation by the patient (P0.1) has also been proposed as an indicator of work of breathing. P0.1 can be based on esophageal pressure or airway pressure. An esophageal pressure P0.1 is invasive but correlates fairly well with work of breathing. An airway pressure P0.1 is non-invasive, but does not correlate nearly as well with work of breathing.
A number of other patents exist for respiratory systems including U.S. Pat. Nos. 6,439,229; 6,390,091; 6,257,234; 6,068,602; 6,027,498, 6,019,732; 5,941,841; 5,887,611; 5,876,352; 5,807,245; and 5,682,881, incorporated herein by reference.
Accordingly, there is a need in the art for a system and method to noninvasively and accurately predict physiologic work of breathing and imposed work of breathing in a patient. The present invention is designed to address this need.